Definitely flush your system with just distilled water 1 or 2 times before putting the new fluid in. If you have been running your system over a year, opening your blocks to see if there's gunk from the colored coolant is recommended.

Would flushing with demineralized water will be fine? Cuz I can't seem to find any distilled water here just yet in Western Australia, tried auto shop, super markets.
Super markets here only sells mineral water, and all I can find from the auto shop is demineralized water..

yeah I've got some demineralized water today, but then I just found out that it's different compared to distilled, and some sites said that distilled is being further purified... So you reckon flushing with demineralized water will be just fine?

I suggest completely dismantle loop and and make sure you get rid of all old coolant. If you flash assembled system some liquid is always trapped. This is something you do not want to skip on, because if two products woudl react, cleaning your parts is MUCH bigger pain, then disassembling.